The invention relates to a method for the cultivation of organisms, especially phototrophic microorganisms in a bioreactor or photobioreactor, wherein bicarbonate ions and carbonate ions or a cation surplus concentration in the culture medium is added. The invention also relates to a tubular bioreactor and photobioreactor wherein the method can be achieved and the use of bioreactors, photobioreactors, tubes and pipes in a method according to the present invention.
Cultivation of phototrophic microorganisms like algae and cyanobacteria in open ponds and raceways is well developed but only a few species can be maintained in traditional open systems. Fully closed photobioreactors (PBR) provide opportunities for monoseptic culture of the microorganisms with specifically adapted culturing conditions. There are different types of photobioreactors like flat plate-PBRs, annular PBRs, e.g. bubble column PBRs and tubular PBRs. These can be further categorized according to orientation of tubes or panels, the mechanism of circulating the culture, the method used to provide light, the type of gas exchange system, the arrangement of the individual growth units, and the materials of construction employed. In the photobioreactor the microorganisms are suspended in a liquid medium. Photobioreactors allow the microorganisms to be illuminated either by natural sunlight or by artificial light sources. A typical photobioreactor is a three-phase system, consisting of a liquid phase, which is the culture medium, the cells as the solid phase and a gas phase. Light, which is the unique feature of photobioreactors is a superimposed radiation field, is sometimes called “the fourth phase” (C. Posten; Eng. Life Sci. 2009, 9, No. 3, pages 165-177).
Of the many designs of closed photobioreactors, devices with tubular solar collectors are the most promising and most widely used commercial systems (E. Molina et al.; Journal of Biotechnology 92 (2001); pages 113-131 and D. Briassoulis et al.: Bioresour. Technol. 101 (2010); pages 6768-6777).
Cultivation of phototrophic microorganisms in photobioreactors for industrial applications is highly dependent on the control of cultivation conditions. The design of the photobioreactors refers mainly to a good availability of light for the microorganisms. Also the pH is one of the major parameters effecting the performance of the microbial population. Production or consumption of CO2 by microorganisms have strong effects on the pH of the cultivation medium.
When illuminated, photosynthesis occurs in the phototrophic microorganisms, enabling the production of carbohydrates from CO2 and H2O. The carbohydrates thus formed enables growth of the organism. This is called autotrophic growth. Due to the photosynthesis process CO2 is consumed by the suspended microorganism, while O2 is produced. The withdrawal of CO2 by the phototrophic microorganisms from the liquid phase causes an increase of the pH of the liquid phase, while the production of oxygen causes an increase of the level of dissolved oxygen in the liquid phase.
In the dark photosynthesis will not occur. Due to respiration O2 will be consumed by the suspended microorganism, while CO2 is produced. The withdrawal of O2 by the microorganisms from the liquid phase will cause a decrease of the level of dissolved O2 in the liquid phase, while the production of CO2 will cause a decrease of the pH.
In order to control pH and dissolved oxygen level of the liquid phase transfer of O2 and CO2 from the liquid phase to the gas phase and vice versa is required. Therefore microorganism cultures in photobioreactors are aerated. In the case of upright liquid containers this is done by injecting gas at the bottom of the reactor. In tubular photobioreactors aeration and degassing is achieved either in the airlift part or, in cases a pump is used for circulating the liquid medium, in a tank which is part of the liquid circuit. This tank is referred to as buffer tank, collection tank, circulation tank or degassing tank. CO2 is fed to upright liquid containers by sparging CO2 or CO2 enriched air directly into the liquid medium at the bottom of the reactor or in the aeration gas flow. In tubular reactors CO2 is commonly injected into the liquid circuit, directly after the air lift part or just before the circulation pump. DE 10 2005 062 726 A1 describes a photobioreactor having devices for circulating the gas in the systems. WO 2010/109108 A1 describes a photobioreactor setting up a two-phase gas/liquid flow in horizontal tubes.
Formation of a gradient of dissolved oxygen and pH between the gas inlet and the gas outlet along the main flow axis of the medium is a problem, especially in tubular reactors. These problems increase with increasing biomass concentrations and increasing light intensities, leading to high volumetric productivities. But high volumetric productivities in term of grams of biomass formed per liter per day are required to improve on the economy of the process.
Of equal importance is enhancing mass transfer along the main axis of the photobioreactor, which is the axial direction in tubes or the upward direction in panel reactors or columns. However, input of mechanical energy by bubbling or pumping is limited to avoid shear stress and cell damage and because energy supply is a major issue in production costs.
The invention is based on the object of providing methods and photobioreactors avoiding the disadvantages of methods and photobioreactors known in the state of the art.
The invention is based also on the object of providing methods and photobioreactors to cultivate phototrophic microorganisms resulting in higher yields of biomass and/or in lower costs.